The present invention relates to a process for the production of aromatic hydrocarbons via the dehydrocyclodimerization of C.sub.2 -C.sub.6 aliphatic hydrocarbons by contacting the aliphatic feedstock with a novel mixture of discrete catalysts.
Dehydrocyclodimerization is a process in which reactants comprising paraffins and olefins, containing from 2 to 6 carbon atoms per molecule are reacted over a catalyst to produce primarily aromatics and H.sub.2, with a light ends by-product, and a C.sub.4.sup.+ nonaromatic by-product. This process is distinct from the more conventional reforming or dehydrocyclization processes where C.sub.6 and higher carbon number reactants, primarily paraffins and naphthenes, are converted to aromatics. The aromatics produced by dehydrocyclization contain the same or a lesser number of carbon atoms per molecule than the reactants from which they were formed, indicating the absence of reactant dimerization reactions. In contast, the dehydrocyclodimerization reaction results in an aromatic product that almost always contains more carbon atoms per molecule than the C.sub.2 -C.sub.6 reactants, thus indicating that the dimerization reaction is an important step in the dehydrocyclodimerization process. Typically, the dehydrocyclodimerization reaction is carried out at temperatures in excess of 260.degree. C. using dual functional catalysts containing acidic and dehydrogenation components. These catalysts include acidic amorphous aluminas which contain metal promoters. Recently, crystalline aluminosilicates have been successfully employed as catalyst components for the dehydrocyclodimerization reaction. Crystalline aluminosilicates generally referred to as zeolites, may be represented by the empirical formula EQU M.sub.2/n O.Al.sub.2 O.sub.3.xSiO.sub.2.yH.sub.2 O
in which n is the valence of M which is generally an element of Group I of II, in particular, sodium, potassium, magnesium, calcium, strontium, or barium and x is generally equal to or greater than 2. Zeolites have skeletal structures which are made up of three dimensional networks of SiO.sub.4 and AlO.sub.4 tetrahedra, corner linked to each other by shared oxygen atoms. The greater the proportion of the SiO.sub.4 species to the AlO.sub.4 species, the better suited the zeolite is for use as a component in dehydrocyclodimerization catalysts. Such suitable zeolites include mordenite and the ZSM variety. In addition to the zeolite component, certain metal promoters and inorganic oxide matrices have been included in dehydrocyclodimeriation catalyst formulations. Examples of inorganic oxides include silica, alumina, and mixtures thereof. Metal promoters such as Group VIII or Group III metals of the Periodic Table, have been used to provide the dehydrogenation functionality. The acidic function may be supplied by the inorganic oxide matrix, the zeolite or both.
As stated previously, aromatics, H.sub.2, a C.sub.4.sup.+ nonaromatic by-product, and a light ends by-product are all products of a dehydrocyclodimerization process. Aromatics are a desired product of the reaction. The aromatic compounds produced by the dehydrocyclodimerization process can be utilized as gasoline blending components. A more important use of the aromatic products is in the production of petrochemicals. The aromatic products may be recovered and separated into pure components such as benzene, toluene, xylenes and so forth for use as precursors for polymers, detergents, chemicals, and the like. Hydrogen is also a desirable product of the process. The hydrogen can be efficiently utilized in hydrogen-consuming refinery processes such as hydrotreating hydrocracking processes. The least desirable product of the dehydrocyclodimerization process are the light ends by-products. The light ends by-products consist primarily of C.sub.1 and C.sub.2 hydrocarbons produced as a result of the cracking of the C.sub.2 -C.sub.6 aliphatic feed molecules and also as a result of the hydrogenation of ethylene. The suppression of this C.sub.2 -C.sub.6 aliphatic feedstock cracking reaction with a subsequent aromatic product yield increase is the particular object to which this application is directed.